Structure-Dynamics Correlation and Its Link to Fragility and Dynamic Heterogeneity

Abstract

Understanding the connection between structure, dynamics, and fragility, the rate at which the relaxation time grows with decreasing temperature, is central to unravelling the glass transition. Fragility is often associated with dynamic heterogeneity, implying that if structure influences dynamics, more fragile systems should exhibit stronger structure dynamics correlations. In this study, we test the generality of this assumption using: Lennard Jones (LJ) and Weeks Chandler Andersen (WCA) systems, where fragility is tuned via density, and a modified LJ (q,p) system, where fragility is varied by changing the potential softness. We define a structural order parameter based on a mean field caging potential and analyse energy barriers at both macroscopic and microscopic levels. While the macroscopic free energy barrier slope correlates with fragility, the microscopic free energy barrier does not show a consistent trend. Instead, it exhibits a strong correlation with a structure dynamics correlation measure obtained from isoconfigurational ensemble simulations. Interestingly, the two systems showing the highest structure dynamics correlation, LJ at rho = 1.1 and the (8,5) model, are respectively the least and most fragile within their classes. These systems exhibit broad mobility distributions, large non Gaussian parameters, yet low four point susceptibilities, suggesting a decoupling between spatial correlation length and mobility contrast. Both systems lie in the enthalpy dominated regime and are close to the spinodal, pointing to mechanical instability as a source of heterogeneity. Our results reveal that structure dynamics correlation is more closely linked to the contrast in individual particle mobility than to the spatial extent of dynamic correlations that typically scale with fragility.